linux-kernel - [PATCH v6 06/20] KVM: VMX: Set FRED MSR intercepts

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Message-ID: <20250821223630.984383-7-xin@zytor.com>
Date: Thu, 21 Aug 2025 15:36:15 -0700
From: "Xin Li (Intel)" <xin@...or.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org
Cc: pbonzini@...hat.com, seanjc@...gle.com, corbet@....net, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, hpa@...or.com, xin@...or.com, luto@...nel.org,
        peterz@...radead.org, andrew.cooper3@...rix.com, chao.gao@...el.com,
        hch@...radead.org
Subject: [PATCH v6 06/20] KVM: VMX: Set FRED MSR intercepts

From: Xin Li <xin3.li@...el.com>

On a userspace MSR filter change, set FRED MSR intercepts.

8 FRED MSRs, i.e., MSR_IA32_FRED_RSP[123], MSR_IA32_FRED_STKLVLS,
MSR_IA32_FRED_SSP[123] and MSR_IA32_FRED_CONFIG, are all safe to
be passthrough, because they all have a pair of corresponding host
and guest VMCS fields.

Both MSR_IA32_FRED_RSP0 and MSR_IA32_FRED_SSP0 are dedicated for
userspace event delivery only, IOW they are NOT used in any kernel
event delivery and the execution of ERETS.  Thus KVM can run safely
with guest values in the two MSRs.  As a result, save and restore of
their guest values are deferred until vCPU context switch and their
host values are restored upon host returning to userspace.

Signed-off-by: Xin Li <xin3.li@...el.com>
Signed-off-by: Xin Li (Intel) <xin@...or.com>
Tested-by: Shan Kang <shan.kang@...el.com>
Tested-by: Xuelian Guo <xuelian.guo@...el.com>
---

Changes in v5:
* Skip execution of vmx_set_intercept_for_fred_msr() if FRED is
  not available or enabled (Sean).
* Use 'intercept' as the variable name to indicate whether MSR
  interception should be enabled (Sean).
* Add TB from Xuelian Guo.
---
 arch/x86/kvm/vmx/vmx.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 42e179f19c23..8e81230be7af 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -4128,6 +4128,43 @@ void pt_update_intercept_for_msr(struct kvm_vcpu *vcpu)
 	}
 }
 
+static void vmx_set_intercept_for_fred_msr(struct kvm_vcpu *vcpu)
+{
+	bool intercept = !guest_cpu_cap_has(vcpu, X86_FEATURE_FRED);
+
+	if (!kvm_cpu_cap_has(X86_FEATURE_FRED))
+		return;
+
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP1, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP2, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP3, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_STKLVLS, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP1, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP2, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_SSP3, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_CONFIG, MSR_TYPE_RW, intercept);
+
+	/*
+	 * MSR_IA32_FRED_RSP0 and MSR_IA32_PL0_SSP (aka MSR_IA32_FRED_SSP0) are
+	 * designated for event delivery while executing in userspace.  Since
+	 * KVM operates exclusively in kernel mode (the CPL is always 0 after
+	 * any VM exit), KVM can safely retain and operate with the guest-defined
+	 * values for MSR_IA32_FRED_RSP0 and MSR_IA32_PL0_SSP.
+	 *
+	 * Therefore, interception of MSR_IA32_FRED_RSP0 and MSR_IA32_PL0_SSP
+	 * is not required.
+	 *
+	 * Note, save and restore of MSR_IA32_PL0_SSP belong to CET supervisor
+	 * context management.  However the FRED SSP MSRs, including
+	 * MSR_IA32_PL0_SSP, are supported by any processor that enumerates FRED.
+	 * If such a processor does not support CET, FRED transitions will not
+	 * use the MSRs, but the MSRs would still be accessible using MSR-access
+	 * instructions (e.g., RDMSR, WRMSR).
+	 */
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_FRED_RSP0, MSR_TYPE_RW, intercept);
+	vmx_set_intercept_for_msr(vcpu, MSR_IA32_PL0_SSP, MSR_TYPE_RW, intercept);
+}
+
 void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 {
 	bool intercept;
@@ -4194,6 +4231,8 @@ void vmx_recalc_msr_intercepts(struct kvm_vcpu *vcpu)
 		vmx_set_intercept_for_msr(vcpu, MSR_IA32_S_CET, MSR_TYPE_RW, intercept);
 	}
 
+	vmx_set_intercept_for_fred_msr(vcpu);
+
 	/*
 	 * x2APIC and LBR MSR intercepts are modified on-demand and cannot be
 	 * filtered by userspace.
-- 
2.50.1